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A low-pressure CO2-based climate-control system has the environmental benefits of CO2 refrigerant but avoids the extremely high pressures of the transcritical CO2 cycle. In the new cycle, a liquid “cofluid” is circulated in tandem with the CO2, with absorption and desorption of CO2 from solution replacing condensation/gas cooling and evaporation of pure CO2. This work compares the theoretical performance of the cycle using two candidate cofluids: N-methyl-2-pyrrolidone and acetone. The optimal coefficient of performance (COP) and refrigeration capacity are discussed in terms of characteristics of the CO2-cofluid mixture. Thermodynamic functions are determined either from an activity coefficient model or using the Soave equation of state, with close agreement between the two approaches. Reductions in COP due to nonideal compressor and heat exchangers are also estimated.

Carbon dioxide (CO2)-based refrigeration systems have been proposed as environmentally benign alternatives to current automotive air conditioners. The CO2 vapor-compression system requires very high operating pressures and complicated control strategies. Recent experimental results indicate that operating pressures comparable to those of current automotive air conditioners can be attained by the inclusion of a secondary carrier fluid (a “co-fluid”), with solution and desolution of the CO2 from the co-fluid substituting for condensation and vaporization of pure CO2. In this work, modeling tools have been developed to optimize the CO2/co-fluid cycle, including the selection of a co-fluid, the CO2/co-fluid ratio (the “loading”), and the operating conditions.